1. Field of the Invention
The embodiment discussed herein is related to a switching power supply apparatus and, more particularly, to a switching power supply apparatus having the function of switching an output voltage and outputting an output voltage after switching and having an overcurrent limiting function.
2. Background of the Related Art
Switching power supply apparatuses convert a commercial AC voltage to a DC voltage having any voltage value and output the DC voltage. Switching power supply apparatuses also accommodate a wide input voltage range. With flyback switching power supply apparatuses an output voltage is insulated from an input commercial source voltage. Flyback switching power supply apparatuses having the function of switching an output voltage and outputting an output voltage after switching and having an overcurrent limiting function are known (see, for example, Japanese Laid-open Patent Publication No. 2009-153234). A switching power supply apparatus having these functions will now be described.
FIG. 9 is a circuit diagram illustrative of a typical example of the structure of a flyback switching power supply apparatus 100. FIG. 10 indicates operational waveforms of important parts of the flyback switching power supply apparatus 100. In the following description the same numeral may be used for representing the name of a terminal and a voltage, a signal, or the like at the terminal.
This switching power supply apparatus includes a control IC 108 for pulse width modulation (PWM) control, which is a control circuit, and includes at least a transformer T, a switching element 17, a diode 19, and a capacitor 20 illustrated in FIG. 9. In this example, a metal oxide semiconductor field effect transistor (MOSFET) is used as the switching element 17. In the following description the switching element 17 is a MOSFET 17.
A voltage of a commercial AC source 1 is supplied to a diode bridge 4 via a common-mode choke coil 2 and an X capacitor 3 included in an input noise filter, and is full-wave-rectified by the diode bridge 4.
A capacitor 5 is connected between an output of the diode bridge 4 and ground and has the function of holding an input voltage for stably supplying energy to an output and the function of absorbing switching noise generated by the switching operation of the MOSFET 17. Furthermore, diodes 6 rectify the voltage of the AC source 1 and supply the voltage to a VH terminal of the control IC 108 via a current limiting resistor 7. The diodes 6 ensure the supply of a source voltage to the control IC 108 at start time. The current limiting resistor 7 limits a current flowing from the AC source 1 to the VH terminal in an emergency such as when a short circuit occurs between the VH terminal and the ground.
A thermistor 9 is connected to a LAT terminal of the control IC 108. When abnormal heat generation of the switching power supply apparatus occurs, the thermistor 9 detects it and provides overheat latch protection to the control IC 108. In addition, a CS terminal of the control IC 108 is connected to a sense resistor 12 via a noise filter including a capacitor 10 and a resistor 11.
A VCC terminal of the control IC 108 is connected to one end of a capacitor 13 and is connected to an auxiliary winding 15 of the transformer T via a diode 14. The capacitor 13 accumulates a starting current supplied at start time from the VH terminal and smooths a voltage supplied from the auxiliary winding 15 and rectified by the diode 14 at PWM control operation time after start. A voltage obtained by accumulating this starting current or a voltage obtained by smoothing the voltage supplied from the auxiliary winding 15 and rectified by the diode 14 is a source voltage of the control IC 108.
One end of a primary winding 16 of the transformer T is connected to the capacitor 5 and the other end of the primary winding 16 of the transformer T is connected to a drain terminal of the MOSFET 17. Furthermore, a source terminal of the MOSFET 17 is grounded via the sense resistor 12. The sense resistor 12 converts an ON-state current of the MOSFET 17 to a voltage signal whose magnitude is proportional to the ON-state current of the MOSFET 17. This voltage signal (current detection signal) is inputted to the CS terminal of the control IC 108 via the noise filter including the capacitor 10 and the resistor 11.
One end of a secondary winding 18 of the transformer T is connected to an anode of the diode 19. A cathode of the diode 19 is connected to one end of the capacitor 20 and an output terminal of the switching power supply apparatus. The other end of the secondary winding 18 of the transformer T is connected to the other end of the capacitor 20 and is grounded. A voltage across terminals of the capacitor 20 is an output voltage supplied to a load. Information regarding this voltage is transmitted from the secondary side to the primary side by a photocoupler 21. A light emitting diode (LED) of the photocoupler 21, a current limiting resistor 22, and two Zener diodes 23 and 24 are connected in series. Both ends of this series circuit are connected to the terminals of the capacitor 20. A collector terminal of a phototransistor of the photocoupler 21 is connected to a FB terminal of the control IC 108. An emitter terminal of the phototransistor of the photocoupler 21 is grounded. As a result, a current proportional to an output voltage on the secondary side (current which is a linear function of an output voltage on the secondary side, more precisely) is converted to an optical signal by the LED. The optical signal is transmitted to the phototransistor and is photoelectric-converted to a signal by the phototransistor. This signal is transmitted to the FB terminal of the control IC 108.
A switch 25 and one (Zener diode 24) of the Zener diodes 23 and 24 connected in series are connected in parallel. The switch 25 is on-off controlled by a standby signal supplied from the load. When the load operates in a normal state, off(open) control of the switch 25 is exercised by a standby signal. When the load is in a standby state, on(close) control of the switch 25 is exercised by a standby signal.
With the switching power supply apparatus using the control IC 108 for PWM control, a voltage obtained by rectifying an input AC voltage is converted to a determined DC voltage via the transformer T by controlling the switching operation of the MOSFET 17.
The control IC 108, which is an IC, detects information regarding a voltage outputted to the load on the secondary side of the transformer T by a signal fed back in the above way to the FB terminal of the control IC 108 via the photocoupler 21.
When the load operates in the normal state, the switch 25 is off-controlled by a standby signal supplied from the load. As a result, a current flowing through the LED is determined by a voltage obtained by subtracting the sum of a forward voltage of the LED, a breakdown voltage of the Zener diode 23, and a breakdown voltage of the Zener diode 24 from an output voltage and the resistance value of the current limiting resistor 22, and is fed back to the FB terminal of the control IC 108. On the other hand, when the load is in the standby state in which only part of the load functions, the switch 25 is on-controlled. As a result, a current flowing through the LED is determined by a voltage obtained by subtracting the sum of a forward voltage of the LED and a breakdown voltage of the Zener diode 23 from an output voltage and the resistance value of the current limiting resistor 22, and is fed back to the FB terminal of the control IC 108. At this time the current flowing through the LED is larger than the current flowing through the LED at the time of the load being in the normal state. Accordingly, the control IC 108 determines that an output voltage is high, and exercises control so as to decrease the output voltage. That is to say, the output voltage is switched to a low voltage and the low voltage is outputted.
Furthermore, the switching power supply apparatus has an overcurrent limiting function. That is to say, if an output power to an output on the secondary side becomes excessively high or if the load is short-circuited, the switching power supply apparatus limits an output power. This overcurrent limiting function is based on a signal which is obtained by converting a drain current flowing through the MOSFET 17 placed on the primary side to a voltage value by the sense resistor 12 connected to the source terminal and which is inputted to the CS terminal of the control IC 108. That is to say, when a voltage value obtained by the sense resistor 12 becomes equal to or greater than a threshold, the control IC 108 changes a gate voltage of the MOSFET 17 from a high (H) level to a low (L) level. By doing so, the control IC 108 exercises control so as to prevent the drain current from becoming equal to or greater than a certain value. This overcurrent limiting function not only prevents damage to a part of the switching power supply apparatus but also prevents a part of the switching power supply apparatus from emitting smoke or producing fire.
The control IC 108 determines an output signal outputted from an OUT terminal by comparing a voltage at the FB terminal and a voltage at the CS terminal directly or indirectly. This output signal controls the on-width of the MOSFET 17. As a result, PWM control of the switching power supply apparatus is exercised. By doing so, power supplied to the load on the secondary side is adjusted.
The above operation of the switching power supply apparatus will be described by reference to the operational waveforms indicated in FIG. 10. In FIG. 10, output power Pout, an output voltage Vout, an output current Iout, a voltage vcc at the VCC terminal of the control IC 108, a signal fb at the FB terminal of the control IC 108, a switching frequency Fsw, and a signal cs at the CS terminal of the control IC 108 are indicated in order from the top.
First it is assumed that the load operates in the normal state and that control is exercised so as to set the output voltage Vout of the switching power supply apparatus to, for example, 20 volts (V). At this time the load undertakes preparation for the standby state. As the output current Iout decreases, the peak values of the output power Pout, the signal fb, the switching frequency Fsw, and the signal cs gradually decrease. When the output power Pout and the output current Iout decreases further, the peak values of the signal fb, the switching frequency Fsw, and the signal cs also decrease further. The load goes into the standby state (switch 25 is on-controlled) at time a. At this time control is exercised so as to change the output voltage Vout from 20 V to 10 V. The switching frequency Fsw decreases to a value at which the voltage vcc for maintaining the operation of the control IC 108 is obtained.
The load then undertakes preparation for returning to the normal state. As the output current Iout and the output power Pout increase, the peak values of the signal fb, the switching frequency Fsw, and the signal cs gradually increase. When the output current Iout reaches a maximum value, the peak values of the signal fb, the switching frequency Fsw, and the signal cs also reach maximum values. When the switching frequency Fsw is a maximum limit value fswmax of the maximum switching frequency, the peak value of the signal cs is limited by an overcurrent limit threshold Vthocp. After that, notice to the effect that the load returns from the standby state to the normal state is given (switch 25 is off-controlled) at time b. At this time control is exercised so as to change the output voltage Vout from 10 V to 20 V.
With switching power supply apparatus of a type which perform switching among two or more output voltages and output an output voltage after the switching, however, the following problem arises. If an overcurrent limit threshold and a maximum switching frequency are constant regardless of an output voltage, an output current value at the time of limiting an overcurrent depends on the magnitude of the output voltage. In the example of FIG. 9, when an output voltage is low (Vout=10 V) (especially during time t), an output current is large (Iout=6.4 amperes (A)). When an output voltage is high (Vout=20 V), an output current is small (Iout=4.4 amperes (A)). Maximum power supplied from the primary side to the secondary side of a transformer is determined regardless of an output voltage. Accordingly, when an output voltage is low, an output current is large. As a result, parts may be selected in accordance with an output current value at the time of an output voltage being low. In particular, a diode on the secondary side, a secondary winding of a transformer, and the like are selected in accordance with a large output current value. However, an output voltage is decreased for the purpose of reducing power consumption in the case of, for example, an apparatus, which is a load, stopping its operation. If an apparatus, which is a load, stops its operation, in many cases operation performed does not need high output power. Originally there is no need to select parts on the assumption that overcurrent operation is performed at the time of decreasing an output voltage. As has been described, with conventional switching power supply apparatus of a type which perform switching among two or more output voltages and output an output voltage after the switching, parts may be selected in accordance with a large output current flowing at the time of an output voltage being low. This is a great waste.